Electrophotographic and ink-jet image forming apparatuses (printers, copiers, facsimile machines, etc.) are generally configured to allow consumable items to be replaced. When an image forming apparatus detects the lives of these replaceable consumable items (also referred to as replaceable items) and determine that the lives have expired and these items should be replaced, the image forming apparatus displays a message that prompts a user or a serviceperson to replace the replaceable items.
For example, in electrophotographic image forming apparatuses, toner cartridges and fusers are replaceable items. The image forming apparatuses detect the lives of such replaceable items. An example of a method for detecting the amount of toner in a toner cartridge is disclosed in PTL 1. This is a method for detecting a change in the amount of toner in the cartridge by detecting electrostatic capacitance. PTL 2 discloses a method for detecting the toner level using an LED and a photo-detector. An example of a method for indirectly estimating the amount of toner is disclosed in PTL 3. This is a method for estimating toner consumption on the basis of image information (pixel count).
However, the replaceable items in the image forming apparatuses vary in dimensional tolerance among replaceable items and in operating environment of the user. This can cause a difference in the degree of degradation among replaceable items, making it difficult to accurately detect the expiration of the lives of the individual replaceable items. If a uniform threshold value for determining the lives is set, the user sometimes cannot use up the replaceable items to the proper expiration of their lives. Although the above methods allow accurate detection of the amount of toner, the methods require a dedicated sensor, leading to a complicated configuration and an increase in cost.